Aircraft gas turbine engines include two or more rotors supported by stationary frames thereof. The rotors carry fan, compressor, and turbine blades having radially outer tips disposed closely adjacent to stationary shrouds for maintaining relatively small blade tip clearances therebetween. Small tip clearances are desired for minimizing the leakage of the motive fluids therepast which decreases efficiency of operation of the engine. Accordingly, the several rotors should be supported in bearings as rigidly as possible for minimizing elastic deflections thereof which can undesirably vary the blade tip clearances as is conventionally known.
In one type of gas turbine engine, a counterrotating power turbine is provided downstream of the core engine for driving forward and aft fan blades either disposed at the forward end of the engine or the aft end of the engine as is conventionally known. Each of the counterrotating turbine rotors includes a respective plurality of rotor blades extending therefrom in predetermined numbers of axial stages. The blade tip clearances at the bottom of the outer rotor blades and at the top of the inner rotor blades are affected by in-plane bending moments of the two rotors due to the various forces encountered during operation of the engine, with it being desirable to control such blade tip clearances by suitably supporting the rotors to stationary frames. Stationary frames are conventionally known and used in various configurations for supporting the power turbine rotors.
However, conventional cantilevered counterrotating rotor support assemblies are suitable for use for blade tip speeds up to about 300 feet/sec (91 meters/sec). Higher speeds substantially increase centrifugal loads and vibratory excitation forces which must be accommodated by the support assembly. For example, the outer blades are subject to compressive column buckling forces and tangential loads which are reacted in part in the outer rotor by hoop stresses. The outer rotor is therefore more subject to radial dynamic motion. And, the inner rotor critical frequencies must be more carefully controlled to avoid undesirable vibratory response. Accordingly, improved support of the power turbine rotors to control deflections due to in-plane bending moments, centrifugal loads, and vibratory response is desired.